The present invention relates in general to amplifier circuits and, more particularly, to a variable gain amplifier circuit.
Amplifiers are widely used in applications such as filters, radio transmitters and receivers, phase locked loops, anywhere it is necessary to amplify an input signal and provide an output signal having a greater or lesser amplitude. Many amplifiers have a fixed gain and provide a constant amplification over a predetermined frequency range of operation. Other applications require some external or dynamic control to set the appropriate amount of amplification for a given input signal.
Variable gain amplifiers are widely used in tunable filters, modulators/demodulators, etc. to provide real-time control over the signal amplification. Prior art variable gain amplifiers, for example those utilizing a Gilbert-gain cell, tend to be complex and consume a large amount of space on an integrated circuit. The Gilbert-gain cell includes two or more voltage-to-current conversion circuits operating in parallel followed by a multiplication stage. One input voltage acts as an amplification control over the other input voltage by way of the multiplication. The product of the currents is then converted back to a voltage that is subject to the variable amplification. The complexity of the prior art variable gain amplifiers tends to reduce its reliability in that a failure of any single component causes a general failure of the amplifier.
Hence, a need exists for a variable gain amplifier having reduced complexity so as to reduce costs and improve reliability.